Currently, information on flat substrates can be displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically written areas to carry ticketing or financial information, however magnetically written data is not visible.
Current flat panel displays use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952, a set of electrical traces is sputtered in a pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive, and currently are limited to applications having long lifetimes.
Fabrication of flexible, electronically written display sheets using conventional nematic liquid crystal materials is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Privacy windows are created from such structures using the scattering properties of polymer dispersed nematic liquid crystals. Polymer dispersed nematic liquid crystals require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic material has the capacity of maintaining one of the given states in the absence of an electric field.
In “Liquid Crystal Dispersions”, World Science, Singapore, 1995, page 408, Paul Drzaic discusses the electrical drive of cholesteric liquid crystal displays. Drzaic also states on page 29 that “The use of gelatin, however, creates a material that is too conductive for practical use in electrically addressed PDLC systems”. Drzaic further states “ . . . actual displays require AC signals to prevent electrochemical degradation.” Subsequent patents support Drzaic's assumptions. Later patents such as U.S. Pat. Nos. 5,251,048, 5,644,330, and 5,748,277 all require AC fields having a net zero field for matrix cholesteric liquid crystal displays to prevent ionic damage to the display. The cited patents have display structures formed using expensive display structures and processes applicable to long life situations that require AC drive schemes.
The drive schemes require that each element be written using alternating electrical fields that provide a net zero field across the display to prevent ionic migration. AC drives require large numbers of power supplies and large numbers of switching elements per line.
Prior art electrical schemes, such as U.S. Pat. No. 5,644,330, require four power supplies to supply +Vc, −Vc, +VR, −VR and ground. Each line output must switch one of three voltages to each line of a matrix display. Conventional bipolar drive schemes, as disclosed in U.S. Pat. No. 5,748,277, require the use of expensive analog switching elements as found in a Supertex HV204 8-Channel High Voltage Analog Switch. One analog switch is required for each voltage applied to each trace of the display. Such expensive chips prohibit low cost commercialization. Even more complex switching schemes have been proposed which increase the number of power supplies and analog switches and are disclosed in other patents, such as U.S. Pat. No. 5,748,277.
U.S. Pat. No. 5,251,048 by Doane et al., discloses a method for driving a cholesteric liquid crystal display using a single chip HD44780 CMOS dot matrix driver integrated circuit available from Hitachi America, Ltd. of Brisbane, Calif. A current model of that chip is HD66712U of the same company. The chips are used to drive nematic liquid crystal display. The Doane et al. patent discloses a method of using nematic liquid crystal drive chips to drive a chiral nematic (cholesteric) liquid crystal display. The table at the bottom of column 8 in the cited reference shows that for each positive voltage, there is an equal and opposite negative voltage for a bipolar drive. The chip for nematic systems is complex due to the use of a bipolar drive system that is also used for cholesteric displays in the Doane patent. Such drives require multiple drive voltages (V1 to V5) to write a display.
Cholesteric displays use expensive conventional flat panel display processes. Consequently, current state of the art requires bipolar voltage drive schemes for cholesteric displays to prevent ionic damage. The bipolar drives require at least two voltages and two separate semiconductor switching elements for each drive line.
Prior art for driving cholesteric liquid crystal displays has been directed towards matrix displays with large numbers of rows and columns, which require multiple drive chips. Display architecture has been directed towards multiple drive chips and power supplies and control logic. Single chip drive systems require multiple voltages that are switched to create bipolar drive schemes. Such architectures are expensive. Certain display applications require few drive lines to present information. It would be useful to drive a simple cholesteric display with a single drive chip using a simple drive method.